The present invention relates to a novel process for producing castings made of aluminum alloy, as well as to a plant for implementing this process.
The current growth of aluminum in the automobile field means that new processes have to be developed, these being tailored to the need to minimize production costs as well as being tailored to mass production (typically, several hundred thousand castings per year and per type of product) and, finally, tailored to the production of castings optimum quality and of increasingly complex geometry, especially due to the constraint of antipollution regulations, resulting in the search for systematic lightening, maximum compactness, optimum performance and integration of functions.
This quality depends both on metallurgical aspects (namely the search for the highest properties with a casting microstructure as fine as possible and as clean as possible in the stressed zones) and on dimensional aspects (in particular, the maximum dimensional precision of all the geometries of the casting, these being critical for the performance of the vehicle.
Certainly, there are a number of processes available for producing automobile castings. However, none of these processes seems to have, at the present time, a combination of characteristics which fully satisfies the combination of requirements mentioned above.
Processes for casting into metal molds, essentially the gravity process and the low-pressure process, are, certainly, economically efficient and deliver a high level of metallurgical and dimensional quality. However, they are unsuitable for producing castings of complex shape.
Thus, the internal shapes are in this case produced by cores of chemically bonded sand, and these processes are very suitable only if it is possible to rapidly insert all these cores after opening the mold and extracting the previous casting. This means that the positioning sequences on the mold have to remain relatively simple, and this therefore proves to be incompatible in some situations, for example in the case of engine blocks or cylinder heads, in which up to twelve cores, or more, have to be put into position along quite complex paths and therefore take an excessively long time to do so.
There are also processes called xe2x80x9csand packingxe2x80x9d processes, especially the process developed by COSWORTH CASTINGS, which were developed in order to meet the abovementioned objectives. However, these processes are very expensive as they have to use a large quantity of chemically bonded sand. Furthermore, in the case of the COSWORTH process, the need to use a special sand of the zircon type, instead of silica sand normally used in foundrywork, also contributes to very high operating costs. Moreover, these processes do not make it possible to achieve the metallurgical quality that may be achieved with the use of molds composed of metal components, which allow the rate of solidification of the aluminum alloy in the most critical zones to be increased to the maximum possible.
There is also a process called the xe2x80x9clost foamxe2x80x9d process which does indeed satisfy the constraints of geometrical complexity and large-scale production. However, the level of metallurgical quality obtained is very much inferior to the current standards in metal-mold casting (gravity or low-pressure casting), so that this process cannot at the present time be envisaged for certain highly stressed applications.
The present invention aims to alleviate the limitations of the prior art and to provide a casting process which better meets the market requirements, particularly the automobile market, and which remains economical to implement.
Another object of the present invention is to provide a casting process using, to at least a substantial extent, physically setting sand, or green sand, which does not raise the particular recycling and environmental problems encountered with chemically setting sands.
Thus, the invention according to a first aspect provides a process for molding a casting made of light alloy such as an aluminum alloy, characterized in that it comprises the successive steps consisting in:
preparing a mold with a print made of physically setting sand,
incorporating a movable closure means in the mold near a feed runner of the mold,
placing the mold in such a way that its feed runner is in the lower part,
connecting the feed runner of the mold to a tube for feeding with a pressurized molten alloy,
filling the mold with said alloy,
before any substantial solidification of the casting, moving the closure means in order to close off the feed runner, then rotating the mold through approximately 180xc2x0 in order to ensure solidification in gravity mode.
Preferred, but nonlimiting, aspects of the process according to the invention are as follows:
provision is furthermore made, between the filling and solidifying steps, for a step of closing off said lower region of the mold followed by separation of the mold from a tube for feeding with molten alloy;
the closure step is carried out less than approximately ten seconds after the end of the filling step;
the rotation step is completed at most 25 seconds, preferably 15 seconds, after the end of closing off;
the rotation step is completed at most 15 seconds, preferably 5 seconds, after the end of closing off;
the rotation step is completed at most 15 seconds, preferably 5 seconds, after the end of filling;
the process uses a mold made of silica sand having a particle size of at least 40 ASS, preferably at least 55 AFS, or at least 80 AFS for excellent surface condition;
a mold consisting of two half-frames is used and the step of mold preparation comprises the steps of molding two half-prints in the two half-frames, positioning molding cores into the two half-frames placed with their prints facing upwardly, and assembling the two half-frames;
the step of assembling the two half-frames results in a mold in a generally horizontal position, and the process furthermore comprises the step consisting in tilting the mold into a generally vertical filling position;
the cores are made of chemically setting sand;
the cores are made of silica sand having a particle size of at least 40 AFS;
provision is furthermore made, after the casting has solidified, for a step of separating the casting from the mold, allowing the print sand and the core sand to be recovered separately;
provision is furthermore made, before the mold filling step, for a step of positioning at least one solid cooler placed at in a region of the mold which is some distance from said feed region of the mold and, after solidification, a step of recovering the cooler or coolers.
According to a second aspect, the invention provides a plant for molding a casting made of light alloy such as an aluminum alloy, characterized in that it comprises:
a mold which can be turned upside down, by rotating it about an essentially horizontal axis, having a runner for feeding with molten alloy and incorporating a means for closing off said runner, and
a mold handling device capable of moving the mold, by rotating it about said horizontal axis, and having a means for actuating said closure means.
Preferred aspects of this plant are as follows:
the handling device has means for moving the mold translationally in the direction of a tube for feeding with molten alloy;
the handling device is also capable of moving the mold, by rotating it about said horizontal axis, between an initial position, on leaving a mold assembly station, and a molding position;
the handling device is capable of moving the mold about a vertical axis in order for it to engage respectively with a conveyor for bringing the mold in, a low-pressure casting furnace fitted with said feed tube, and a conveyor for taking the mold away.
Finally, according to a third aspect, the invention provides a mold intended for casting a casting made of light alloy such as an aluminum alloy, the mold being provided with a runner for feeding with pressurized molten alloy, the mold being characterized in that it is mounted so as to rotate about an essentially horizontal axis, so as to be able to be turned upside down after filling, and in that it comprises a means for mechanically closing off said feed runner.
Preferred, but optional, aspects of this mold are as follows:
the mold has at least one print made of physical setting sand and said mechanical closure means comprises a metal plate incorporated in the print and guided directly by the latter;
the mold comprises a blind hole terminating in line with one edge of said metal plate and capable of housing a rod of a means for actuating said plate;
said plate has at least one guiding appendage which, in an initial position of said plate, engages in an opposite print of the mold.